George Osborne, William Hague and Theresa May join forces to declare that
allowing gay marriage "is the right thing to do at the right time".

In an unprecedented joint letter amid a Cabinet split before tomorrow’s parliamentary vote, the ministers, who hold the three great offices of state, have written a letter to The Telegraph saying that “attitudes to gay people have changed”.

Their intervention represents a late attempt to persuade other Conservatives to drop their deep-seated opposition to legalising gay marriage. The issue threatens to divide the Conservative Party with more than half of MPs and about four members of the Cabinet refusing to support David Cameron’s decision to champion the move.

David Jones, the Welsh Secretary, yesterday emerged as an opponent to gay marriage. In a letter to a constituent in March last year, he wrote: “I believe that marriage is an institution ordained to sanctify a union between a man and a woman.” Letters written by six government whips, all opposing gay marriage, were also released by campaigners. Although the measure is likely to pass through the Commons with the support of Labour and Liberal Democrat MPs, the Prime Minister is facing increasingly personal attacks from some Conservative MPs over the legislation.

Downing Street sources insisted yesterday that MPs would be free to vote as they wished and that no pressure was being put on Conservatives to back the legislation. But there will be a second technical vote on the timetable for the introduction of gay marriage laws, which Conservative MPs will be forced to support and which may cause a rebellion. Senior sources said it would be “very serious” if any members of the Government refused to back the programme.

Although the Prime Minister will not publicly speak in favour of gay marriage, the letter from the most senior members of the Cabinet indicates that those around Mr Cameron remain committed to the need for the new laws. The letter says: “Civil partnerships for gay couples were a great step forward, but the question now is whether it is any longer acceptable to exclude people from marriage simply because they love someone of the same sex.

“Marriage has evolved over time. We believe that opening it up to same-sex couples will strengthen, not weaken, the institution.

“Attitudes towards gay people have changed. A substantial majority of the public now favour allowing same-sex couples to marry, and support has increased rapidly. This is the right thing to do at the right time.”

Chris Grayling, the Justice Secretary, who was rumoured to be against gay marriage, wrote an article for a gay magazine yesterday backing the legislation.

“The state’s job is to recognise the commitment between a couple to spend the rest of their lives together and not to judge who that couple are,” he wrote in Attitude magazine. He said it is a “sensible next step” in the evolution of social attitudes.

Some Cabinet ministers have publicly stated their opposition. Mr Jones, the Welsh Secretary, is said to be ready to vote against the Prime Minister, with Owen Paterson, the Environment Secretary. Others, including Philip Hammond, the Defence Secretary, are set to abstain or be absent for the vote. Iain Duncan Smith, the Work and Pensions Secretary, is understood to be considering voting to back the reform.

Last November, John Randall, the deputy chief whip, told a constituent he feared that marriage was “taken too lightly by too many” and he did not want any vows to be “devalued”. “I have not yet received any persuasive arguments why the existing civil partnership does not give equality and what material advantages would be gained for same-sex couples in having a 'civil marriage’ rather than a 'civil partnership’ other than in the name,” he said.

He said he was “far from happy” about the proposals, adding: “I think that it is most likely if and when legislation eventually comes before Parliament that I would oppose it.” Mark Lancaster, another government whip, wrote to a constituent last year that he was quite clear that this is not an issue that I support and would not vote for it”.

David Evennett, the MP for Bexleyheath and Crayford, also a whip, said he was “a strong supporter of traditional marriage and as such I will not be voting for any changes”.

Robert Goodwill said he intended to vote against the Bill, while Stephen Crabb said in December: “I share the views of a great many Christians and people of other faiths who have a strong conception of marriage as a union between a husband and wife.”

A poll has found that the Conservatives risk losing more votes than it would win by introducing gay marriage. The ComRes poll for ITV News found that just over a third of respondents thought it would make the Tories less appealing to them as a voter, while only 15 per cent said it would make it more so.

Labour MPs will be ordered to support the timetable, although they will be free to vote with their consciences on the main Bill itself.

Ed Miliband, the Labour Leader, is urging his MPs to deliver a “decisive majority” in the Commons in favour of gay marriage.

However, Labour sources expect up to 25 MPs, most of whom have religious convictions, to oppose the Bill. Only a handful of Roman Catholic Liberal Democrat MPs are expected to vote against.

While Mr Miliband will support the Bill, he has criticised the Government for including legal protections for the Church of England so that it will not be legally required to conduct gay marriages.

Mr Miliband, who does not believe in God, has said the special exclusion for the Church of England and Church in Wales was “disappointing”.

The Church of England, which claims that the legal protections in tomorrow's legislation will be open to challenge in the European courts, remains deeply opposed to gay marriage.

Speaking on his first day in office, the new Archbishop of Canterbury, the Most Rev Justin Welby, underlined his opposition to the Government’s plans and said the Church had “made its views clear”.

The Archbishop insisted he was not on a “collision course” with the Government but said there were “issues with the way it is going forward.”